How to get wind to affect hair particles with group or object render
Finally I understand what @Brenticus was getting at when he kept using the word "deform"! If you use either the "Object" or "Group" methods to render the particles, you can in fact affect the rotation and/or location of the objects. Here is a video showing that much:



But what is missing is (in this case) the grass stems deforming along the hair particles. Here is the comparison:



So the answer to my original question is outlined below in my previous answer. I only need to add that you must rotate the object(s), generally 90 degrees along the y-axis because "hair particles use the global +X axis as 'up'". That is the case for either method.

That method works if it's not important to deform the object(s) along the hair particles, and you get the benefits of "Group" options, which are quite powerful. However, in making the comparison videos I noticed that the render time was considerably longer using that method, which is another thing to keep in mind.

If it's important for the objects to deform along the hair particles, then I strongly suspect Brenticus is right and that using a particle instance modifier as per this post is just not the best but the only way of doing it. I spent several hours looking for a way to get group objects to deform and could not.

Previous answer (for Group and Object render method)

The first part is the "initial orientation" which I had set to "Global Z" so that the grass grows straight up. Changing this to "Velocity Hair" was one necessary change. 



The second part is in the force field wind settings. Under "Effect point", I had deselected "Location" because I only want to affect the rotation of the grass stems. I guess that thinking was faulty. Selecting "Location" got the grass moving. 



So I now have the wind affecting the grass.